/**
 * Copyright (C) 2011 Whisper Systems
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */
package com.appspot.eusms.protocol;

import org.whispersystems.textsecure.util.Base64;

import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
import java.security.SecureRandom;

/**
 * Calculates prefixes that identify a message as
 * being part of an encrypted session.  The idea was to
 * make calculating and identifying these prefixes somewhat
 * expensive, so that filtering them en-mass would come at a cost.
 *
 * @author Moxie Marlinspike
 */

public abstract class WirePrefix {

    private static final int HASH_ITERATIONS = 1000;
    private static final int PREFIX_BYTES = 3;
    public static final int PREFIX_SIZE = 4;

    public abstract String calculatePrefix(String message);

    public static boolean isKeyExchange(String message) {
        return verifyPrefix("?TSK", message);
    }

    public static boolean isEncryptedMessage(String message) {
        return verifyPrefix("?TSM", message);
    }

    public static boolean isPreKeyBundle(String message) {
        return verifyPrefix("?TSP", message);
    }

    public static boolean isEndSession(String message) {
        return verifyPrefix("?TSE", message);
    }

    public static String calculateKeyExchangePrefix(String message) {
        return calculatePrefix(("?TSK" + message).getBytes(), PREFIX_BYTES);
    }

    public static String calculateEncryptedMesagePrefix(String message) {
        return calculatePrefix(("?TSM" + message).getBytes(), PREFIX_BYTES);
    }

    public static String calculatePreKeyBundlePrefix(String message) {
        return calculatePrefix(("?TSP" + message).getBytes(), PREFIX_BYTES);
    }

    public static String calculateEndSessionPrefix(String message) {
        return calculatePrefix(("?TSE" + message).getBytes(), PREFIX_BYTES);
    }

    private static boolean verifyPrefix(String prefixType, String message) {
        if (message.length() <= PREFIX_SIZE)
            return false;

        String prefix = message.substring(0, PREFIX_SIZE);
        message = message.substring(PREFIX_SIZE);

        String calculatedPrefix = calculatePrefix((prefixType + message).getBytes(), PREFIX_BYTES);

        assert (calculatedPrefix.length() == PREFIX_SIZE);

        return prefix.equals(calculatedPrefix);
    }

    private static String calculatePrefix(byte[] message, int byteCount) {
        try {
            MessageDigest md = MessageDigest.getInstance("SHA1");
            byte[] runningDigest = message;

            for (int i = 0; i < HASH_ITERATIONS; i++) {
                runningDigest = md.digest(runningDigest);
            }

            return Base64.encodeBytes(runningDigest, 0, byteCount);
        } catch (NoSuchAlgorithmException e) {
            throw new AssertionError(e);
        }
    }

    public static String calculateEncryptedMmsSubject() {
        try {
            byte[] postfix = new byte[6];
            SecureRandom.getInstance("SHA1PRNG").nextBytes(postfix);

            String postfixString = Base64.encodeBytes(postfix);
            String prefix = calculatePrefix(postfixString.getBytes(), 6);

            return prefix + postfixString;
        } catch (NoSuchAlgorithmException e) {
            throw new AssertionError(e);
        }
    }

    public static boolean isEncryptedMmsSubject(String subject) {
        if (subject.length() < 9)
            return false;

        String prefix = subject.substring(0, 8);
        String postfix = subject.substring(8);

        String calculatedPrefix = calculatePrefix(postfix.getBytes(), 6);
        return calculatedPrefix.equals(prefix);
    }
}
